The present invention relates to electronic circuits and, more particularly, to voltage regulators including a ballast or pass resistor and a current limiter.
Certain voltage regulators may be equipped with a so-called pass or ballast transistor positioned in series with a regulator input and output. These voltage regulators operate by using a potential barrier voltage reference, for example, and the output voltage is regulated by feedback. Such regulators are well known in the art, particularly for implementing integrated regulators.
If a short circuit occurs between the regulator output terminals, the input voltage of the regulator appears at the terminals of the ballast transistor. This may result in an excessive current flowing through the ballast transistor, potentially resulting in the destruction thereof. One approach to limiting this current involves adding a current limiting circuit including a low value resistor (e.g. 1 ohm) in series with the ballast transistor.
High density complementary metal-oxide semiconductor (xe2x80x9cHCMOSxe2x80x9d) technology has been developed for increasing the integration density of integrated circuits. An integrated regulator obtained by using HCMOS technology must supply a regulated voltage of 3.3 V for an input voltage of 5 V. The ballast transistor tolerates a maximum voltage of 5 V between its terminals. In the case of a short circuit at the regulator output, protection of the ballast transistor must be very rapid and efficient.
The present invention provides a rapid and efficient protection to a ballast transistor of a voltage regulator.
This and other objects, features, and advantages in accordance with the present invention are provided by a voltage regulator having a current limiter for regulating an input voltage and for delivering an output voltage. The voltage regulator may be implemented using HCMOS technology, for example. The voltage regulator may include a voltage regulating circuit including an amplifier circuit and a first feedback circuit. The amplifier circuit may include a ballast transistor, and the first feedback circuit supplies a first feedback voltage representative of the voltage regulated by the ballast transistor to the amplifier circuit for comparison with a reference voltage.
The voltage regulator may further include a limiting circuit including a current limiting transistor connected in series with the ballast transistor and an output of the voltage regulator, a controller for biasing the current limiting transistor, and a second feedback circuit supplying a second feedback voltage representative of a voltage at the output to the controller. The controller makes the current limiting transistor operate between saturation and blocking conditions, depending on whether the second feedback voltage is above or below a predetermined threshold voltage. The first and second feedback circuits may each include a resistor-equipped voltage divider.
The voltage regulator may also include a differential amplifier having first and second inputs of opposite polarity, the first input receiving the reference voltage and the second input receiving the first feedback voltage. An output voltage of the differential amplifier controls the ballast transistor. The second feedback voltage corresponding to the current limiting part is supplied to a third input having an opposite polarity of the first input. A fraction of the output voltage may be connected to the controller to provide a control voltage therefor. The controller may be a comparator, for example, having a first input receiving the reference voltage, a second input receiving the control voltage, and an output connected to the current limiting transistor. The output voltage fraction may be provided by a transistor sized in accordance with the ballast transistor.